1. Field of the Invention
The present invention relates to an electrodynamic loudspeaker for reproducing sound, and more particularly to an electrodynamic loudspeaker in which the weight of the magnetic circuit is reduced.
2. Description of the Related Art
In an electrodynamic loudspeaker, a vibrator including a diaphragm, a voice coil (including a coil bobbin and a voice coil wound around the coil bobbin) and a damper vibrates with respect to a magnetic circuit fixed to a frame, thereby reproducing sound. For example, with an inner-magnet-type magnetic circuit, the magnetic circuit includes a magnet, a pole and a yoke, with the voice coil of the vibrator being placed in the magnetic gap defined between the pole and the yoke. When a sound signal is applied through the voice coil, the vibrator receives a driving force therefrom and vibrates. The pole and the yoke forming the magnetic path of the magnetic circuit are made of a soft magnetic material such as iron, and the weight of the magnetic circuit accounts for a large portion of the total weight of the electrodynamic loudspeaker. In view of this, there have been conventional approaches to reduce the weight of an electrodynamic loudspeaker by improving the shape, etc., of the magnetic circuit thereof.
Loudspeakers with reduced weight include those using a repulsion-type magnetic circuit in which two magnets are placed on the upper and lower surfaces of a pole with their like poles facing each other, and those using a magnetic circuit in which a pole is placed on each of the two magnetic pole surfaces of a single magnet. In these magnetic circuits, the yoke for forming the magnetic path and the magnetic gap is omitted in order to reduce weight, thereby forming “open-type” magnetic circuits. The voice coil is placed so as to face the pole in which the magnetic flux is concentrated. Then, an open-type magnetic circuit with no yoke has a problem in that the magnetic flux density is decreased in the position of the voice coil. The distance over which magnetic force lines pass through the air increases, thus increasing the magnetic resistance of the magnetic path, and the magnetic force lines are no longer concentrated in the position of the voice coil, whereby the magnetic flux density in the position of the voice coil is further decreased. Thus, the reduction in the weight of the magnetic circuit results in a decrease in the efficiency of the electrodynamic loudspeaker, thereby failing to obtain desirable acoustic characteristics.
In view of such a problem resulting from a reduction in weight of a magnetic circuit, there is a conventional electrodynamic loudspeaker in which a thin-walled annular core of amorphous metal (magnetic body) is provided on the outer surface of a voice coil in an attempt to concentrate the magnetic force lines in the thin-walled annular core of amorphous metal (Japanese Patent Publication for Opposition No. 6-71360). In another conventional electrodynamic loudspeaker, a magnetic body such as iron is used as the core of a voice coil, and a conductive material such as copper or aluminum is adhered to the core by means of plating (Japanese Laid-Open Patent Publication No. 6-233379).
However, the conventional electrodynamic loudspeakers cannot realize both a reduction in weight and desirable acoustic characteristics. Specifically, if a magnetic body is provided on the outer surface of a voice coil, or if a magnetic body is used as the core of a voice coil, the voice coil, which is placed so as to face the pole in which magnetic force lines are concentrated, cannot sufficiently vibrate when a sound signal is applied through the voice coil because the magnetic body of the voice coil near the pole is grabbed by the magnetic force from the DC magnetic flux in the magnetic circuit. If the coil bobbin is not sufficiently strong, the magnetic force acting upon the magnetic body may cause the coil bobbin and the pole to be in contact with each other, which may result in a defective operation. As a result, with the conventional electrodynamic loudspeakers, a reduction in weight decreases the efficiency, thereby failing to realize desirable acoustic characteristics.